Digital holography has been experiencing a rapid growth over the last several years, together with the availability of cheaper and better digital components as well as more robust and faster reconstruction algorithms, to provide new microscopy modalities that improve various aspects of conventional optical microscopes. Among many other holographic approaches, Digital In-Line Holographic Microscopy (DIHM) provides a simple but robust lens-free imaging approach that can achieve a high spatial resolution with e.g., a numerical aperture (NA) of ˜0.5. To achieve such a high numerical aperture in the reconstructed images, conventional DIHM systems utilize a coherent source (e.g., a laser) that is filtered by a small aperture (e.g., <1-2 μm); and typically operate at a fringe magnification of F>5-10, where F=(z1+z2)/z1; z1 and z2 define the aperture-to-object and object-to-detector vertical distances, respectively. This relatively large fringe magnification reduces the available imaging field-of-view (FOV) proportional to F2.
In an effort to achieve wide-field on-chip microscopy, the use of unit fringe magnification (F˜1) in lens-free in-line digital holography to claim an FOV of ˜24 mm2 with a spatial resolution of <2 μm and an NA of ˜0.1-0.2 has been demonstrated. See Oh C. et al. On-chip differential interference contrast microscopy using lens-less digital holography. Opt Express.; 18(5):4717-4726 (2010) and Isikman et al., Lensfree Cell Holography On a Chip: From Holographic Cell Signatures to Microscopic Reconstruction, Proceedings of IEEE Photonics Society Annual Fall Meeting, pp. 404-405 (2009), both of which are incorporated herein by reference.
This recent work used a spatially incoherent light source that is filtered by an unusually large aperture (˜50-100 μm diameter); and unlike most other lens-less in-line holography approaches, the sample plane was placed much closer to the detector chip rather than the aperture plane, i.e., z1>>z2. This unique hologram recording geometry enables the entire active area of the sensor to act as the imaging FOV of the holographic microscope since F˜1. More importantly, there is no longer a direct Fourier transform relationship between the sample and the detector planes since the spatial coherence diameter at the object plane is much smaller than the imaging FOV. At the same time, the large aperture of the illumination source is now geometrically de-magnified by a factor that is proportional to M=z1/z2 which is typically 100-200. Together with a large FOV, these unique features also bring simplification to the set-up since a large aperture (˜50 μm) is much easier to couple light to and align.
However, a significant trade-off is made in this recent approach. To wit, the pixel size now starts to be a limiting factor for spatial resolution since the recorded holographic fringes are no longer magnified. Because the object plane is now much closer to the detector plane (e.g., z2˜1 mm), the detection NA approaches ˜1. However, the finite pixel size at the sensor chip can unfortunately record holographic oscillations corresponding to only an effective NA of ˜0.1-0.2, which limits the spatial resolution to <2 μm. While, in principle, a higher spatial density of pixels could be achieved by reducing pixel size at the sensor to e.g., <1 μm, this has obvious technological challenges to use in a large FOV.